Microwave Sounding

1. Microwave Sounding • Ben Kravitz • October 29, 2009

2. What is Microwave Sounding? Passive sensor in the microwave to measure temperature and water vapor Technique was pioneered by Ed Westwater (c. 1978)

3. Microwave Gamma X-Rays Ultraviolet Visible Infrared Microwave (and beyond): “Extremely” High Frequency “Super” High Frequency “Ultra” High Frequency “Very” High Frequency High Frequency Medium Frequency Low Frequency “Very” Low Frequency Voice Frequency “Super” Low Frequency“Extremely” Low Frequency { {

4. Main Purpose • Measure total integrated water content in an atmospheric column (both vapor and liquid) • Measure a (coarse) vertical profile of atmospheric water

5. We will begin with ground-based microwave radiometers

6. Vocabulary • Total column water vapor = precipitable water • Total column liquid water (in a cloud) = liquid water path (LWP)

7. Why do we care about column water vapor?

8. Why do we care about column water vapor? • Water vapor is the most abundant greenhouse gas in the atmosphere • Essential for weather forecasting models • Atmospheric propagation delays

9. Why do we care about column liquid water?

10. Why do we care about column liquid water? • In a cloud, the amount of liquid water is very important in determining optical depth

11. Optical Depth note the wavelength dependence! Iλ=Iλ,0e-τλm Iλ,0 = incident solar radiation (at the top of the atmosphere) Iλ = solar radiation that reaches the surface τλ=optical depth m = atmospheric mass (how much of the atmosphere the radiation is passing through) Optical depth describes the attenuation of solar radiation as it passes through the atmosphere

12. Optical Depth Understanding optical depth is ESSENTIAL to understanding the radiation budget

13. Until Westwater invented the technique of microwave sounding, we were using radiosondes to determine atmospheric humidity

14. Problems with Radiosondes

15. Problems with Radiosondes • Radiosondes tend to drift (move with the wind) • Radiosondes are not released often enough • Radiosonde measurements of humidity are sometimes suspect

16. Passive Microwave Radiometers • VERY accurate in determining column integrated water quantities • Get complete measurements every 20 minutes • Operate in nearly all conditions, regardless of weather

17. Passive Microwave Radiometers • Can also be used to give vertical profiles of liquid water and water vapor, albeit at very poor resolution

18. Resonant Frequency(of water) • Frequency at which vibration can be induced in water molecules • 22.235 GHz

19. Procedure • Point the radiometer in a given direction and calculate atmospheric mass • Measure the amount of radiation at the frequency to which you tune the radiometer • Plug it into a simple radiative transfer equation which gives you total optical depth in the column

20. Merged Sounding • Pioneered by Dr. Miller (and colleagues) • Take the column profiles and feed them into the weather forecasting model run by the European Center for Medium Range Weather Forecasting (ECMWF) • Use this to correct radiosonde data

21. Humidity Profiling

22. Large peak at 22.235 GHz

23. Radiation at 22.235 GHz Water molecule A Water molecule B Microwave Radiometer tuned to 22.235 GHz

24. The microwave radiometer will not see much of molecule A This is not a good way to design a profiler

26. Radiation at frequency higher than 22.235 GHz Water molecule A Water molecule B Microwave Radiometer tuned to different frequency

27. The microwave radiometer at this frequency DOES see molecule A We do this at multiple frequencies, and we can get a vertical profile

28. Weighting Functions Each channel (frequency) has a function that tells it how much to weight each elevation Deriving these is as much an art as a science and requires a lot of experience doing this sort of thing

29. Weighting Functions

30. What frequencies are used? • Most microwave radiometers have 5 channels devoted to water • Each group has their own preference as to which frequencies they use • Most important ones: 20.6 GHz, 22.235 GHz, and 31.65 GHz

31. Line Width All instruments are imperfect and have an aperture of some kind. If you want to measure, say, 22.235 GHz, you cannot measure exactly that frequency and no others. You might actually measure something like 22.235±0.5 GHz. This 0.5 GHz is called the line width. The line width affects the shape of the absorption curve.

32. Line Width 20.6 GHz frequency is relatively insensitive to line width

33. If we measure at 20.6 GHz, we can be sure that the line width is not introducing a source of error into our measurements. There is a similar feature at 24.4 GHz.

34. relative minimum at 31.65 GHz

35. The 31.65 GHz frequency shows a drop-off of absorption. This reduction of absorption is greater for water vapor than for liquid water. By measuring at this frequency, we can differentiate between water vapor and liquid water.

36. Resolution The resolution of microwave humidity profilers is about 1 km. This is not at all useful for a large majority of clouds.

37. We’ve discussed the 5 channels in the 20-32 GHz range Most microwave radiometers have 12 channels total There are 7 more channels in the 48-71 GHz range

38. Oxygen Notice the reduction in absorption with height

39. Absorption of microwave energy by oxygen is very dependent upon temperature We can use these 7 channels to profile temperature

40. For very dry conditions, the 31.65 GHz channel is not very good at distinguishing water vapor from liquid water. For this reason, some radiometers also take measurements at 183 GHz. This frequency is very sensitive to liquid water.

42. Calibration • Microwave radiometers have a strong tendency to drift • They need to be calibrated quickly and often

43. Atmospheric Mass To a good approximation, m≈sec(θ) θ = zenith angle As zenith angle increases (moves away from the vertical), so does atmospheric mass

44. Tipping Angle Distance to top of atmosphere increases with zenith angle, so the amount of atmosphere between the radiometer and the sun grows with angle m increases θ=0° ⇒ m=1 θ=90° ⇒ m=∞

45. Langley Plot log plot 0.1 0.01 0.001 best linear fit τ 1 2 3 4 5 m

46. Atmospheric Mass • We define that for m=0, τ should equal 0 • We can’t actually measure anything for m<1 • To get τ for m=0, we use the Langley plot and extrapolate backwards to m=0

47. Langley Plot 0.1 0.01 0.001 best linear fit τ τ at m=0 1 2 3 4 5 m

48. Calibration • Usually, τ≠0 at m=0 • This tells us how much we need to correct our measurements • This is how we calibrate the radiometer

49. Space-based microwave radiometers

50. NASA has had microwave sounders in orbit since 1978 (MSU, which flew on TIROS-N) Microwave sounders have given us a very long satellite-based temperature record Resolution and number of channels has dramatically improved since then Using microwave sounders to measure temperature and humidity is an idea that is losing currency with the advent of GPS radio occultation (which we’ll talk about later in the class)